The present disclosure relates to a power receiving device, a power transmitting device, a wireless power transfer system, and a wireless power transfer method that detect the presence of a conductor such as a metal.
Recently, wireless power transfer systems, which contactlessly transmit electric power (transmit electric power by wireless), are being actively developed. In the wireless power transfer unit, an alternating signal is input to a power transmitting coil in a power transmitting device to generate an alternating magnetic field; a power receiving coil in a power receiving device receives the alternating magnetic field, and an alternating signal is retrieved to transfer (supply) and electric power by wireless.
FIG. 1 schematically illustrates an example of the structure of a wireless power transfer system.
The wireless power transfer system 1 in FIG. 1 includes a power transmitting device 10 on a primary side and a power receiving device 20 on a secondary side.
The power transmitting device 10 includes, as an example, a power transmitting coil 11, a resistive element 12, and an alternating power supply 13, which outputs alternating signals. In the power transmitting device 10, an alternating signal is input from the alternating power supply 13 to the power transmitting coil 11 to generate an alternating magnetic field.
The power receiving device 20 includes, as an example, a power receiving coil 21, a capacitor 22 connected in parallel to the power receiving coil 21, a rectifying and smoothing unit 23, a regulator 24, and a power receiving target 25.
In the power receiving device 20, the power receiving coil 21 receives the alternating magnetic field generated in the power transmitting device 10 by, for example, a magnetic resonance method and an alternating signal is retrieved through a resonance circuit including the power receiving coil 21. The retrieved alternating signal is converted into a direct-current signal by being rectified and smoothed by the rectifying and smoothing unit 23. The regulator 24 uses the converted direct current signal to generate a constant voltage, and supplies the generated constant voltage to the power receiving target 25 such as a battery.
Usually, the wireless power transfer unit also carries out communication between the power transmitting device and the power receiving device for control purposes while transferring electric power. As the communication method, alternating signals to be transferred are often subjected to amplitude modulation (for example, amplitude shift keying (ASK)) during data communication.
FIG. 2 illustrates an example of a structure in which data is transmitted from the power transmitting device 10 (primary side) to the power receiving device 20 (secondary side). In FIG. 2, elements that are the same as in FIG. 1 will be given the same reference numerals.
In the power transmitting device 10, a parallel circuit formed with a resistive element 14R and a switch 14S is added to the data transmission structure in FIG. 1 between the power transmitting coil 11 and the alternating power supply 13, and a communication and control unit 16 is also included.
When data is transmitted from the power transmitting device 10, the amplitude of an alternating signal generated by the alternating power supply 13 is directly changed. Specifically, to perform amplitude modulation of the alternating signal generated by the alternating power supply 13, the communication and control unit 16 makes a switchover between the open state and closed state of the switch 14S according to the transmission data string (baseband signal).
In the power receiving device 20, a demodulation circuit 26 and a communication and control unit 27 are added to the data reception structure in FIG. 1.
When the power receiving device 20 receives data, the amplitude modulated alternating signal is rectified and smoothed by the rectifying and smoothing unit 23, the resulting direct-current signal is demodulated by the demodulation circuit 26, and a reception data string (baseband signal) is extracted. The reception data string is analyzed by the communication and control unit 27.
FIG. 3 illustrates an example of a structure in which data is transmitted from the power receiving device 20 (secondary side) to the power transmitting device 10 (primary side). In FIG. 3, elements that are the same as in FIGS. 1 and 2 will be given the same reference numerals.
In the power receiving device 20, a series circuit formed with a resistive element 28R and a switch 28S is added to the data transmission structure in FIG. 1 in parallel to the power receiving coil 21, and the communication and control unit 27 is also included.
When data is transmitted from the power receiving device 20 to the power transmitting device 10, the so-called load modulation method is usually used. Specifically, to change the value of the load resistive component parallel to the power receiving coil 21, the communication and control unit 27 makes a switchover between the open state and closed state of the switch 28S according to the transmission data string (baseband signal). The alternating signal output from the power transmitting device 10 is thereby amplitude modulated, enabling the power transmitting device 10 to observe the transmission data string transmitted from the power receiving device 20.
In the power transmitting device 10, a modulation circuit 15 and the communication and control unit 16 are added to the data transmission structure in FIG. 1.
When the power transmitting device 10 receives data, the amplitude modulated alternating signal received by the power transmitting coil 11 is demodulated by the modulation circuit 15 and a reception data string (baseband signal) is extracted. The reception data string is analyzed by the communication and control unit 16.
Another possible communication method is to use a short-distance wireless communication standard such as Bluetooth® or ZigBee® with a frequency different from the frequency of the supplied alternating signal. The above communication method, in which the supplied alternating signal is amplitude modulated, is usually used because the number of parts used can be reduced, the hardware can be simplified, and the number of frequencies used can be reduced to one.
The power receiving device 20 usually uses electric power received from the power transmitting device 10 to operate digital circuits, a microcomputer, and other elements intended for communication and control. When the power receiving coil 21 is moved apart from the power transmitting coil 11, therefore, the power supply of the power receiving device 20 is turned off.
FIG. 4 is a flowchart illustrating an ordinary example of the operation of the digital circuits, the microcomputer (control unit), and the like when the power transmitting device (primary side) and power receiving device (secondary side) perform communication for control purposes to transmit electric power (to, for example, a battery).
In the examples in FIGS. 2 and 3, the control unit is equivalent to the communication and control unit 16 in the power transmitting device 10 and to the communication and control unit 27 in the power receiving device 20.
When the user or another person turns on the power supply of the primary side (step S1), the primary side carries out object detection (step S2). When the user makes the secondary coil face the primary coil, the secondary side is detected by the primary side as an object (steps S5 and S6). The primary side decides whether any object has been detected (step S3). If some kink of object has been detected, the sequence proceeds to step S4. If no object has been detected, the sequence returns to step S2.
If an object has been detected in step S3, the primary side starts to transmit electric power (step S4). The power supply of the secondary side is turned on by the electric power transmitted from the primary side (step S7).
Next, the primary side and the secondary side mutually communicate to exchange their ID numbers (identification information) (step S8). To assure safety, the primary side and secondary side then carry out mutual authentication by using authentication keys (step S9). The primary side decides, from the authentication result, whether the secondary side is a correct remote party (step S10). If the secondary side is not a correct remote party, the primary side suspends the power transmission and the secondary side stops the operation without performing charging (step S14). It suffices to perform the mutual authentication only once after the power supplies of the primary side and secondary side have been turned on.
If communication between the primary side and the secondary side is discontinued or fails in step S8 or S9, the primary side suspends the power transmission (step S15) and the power supply of the secondary side is turned off, after which the sequence returns to step S2.
If the primary side can decide that the secondary side is a correct remote party, the primary side transmits electric power to the secondary side to allow the secondary side to charge the battery. If metallic foreign matter enters a clearance between the coil on the primary side and the coil on the secondary side, an eddy current flows in the metallic foreign matter, generating heat. To prevent this, the secondary side carries out metallic foreign matter detection before starting charging.
That is, if the primary side decides that the secondary side is a correct remote party in step S10, the primary side or secondary side carries out metallic foreign matter detection (step S11); when the secondary side carries out metallic foreign matter detection, it uses the electric power transmitted from the primary.
Then, the primary side or secondary side decides whether metallic foreign matter has been detected (step S12).
If no metallic foreign matter has been detected, the secondary side supplies the electric power received from the primary side to the power receiving target to perform battery charging (step S13). It is difficult to know a time at which metallic foreign matter intrudes. To perform metallic foreign matter detection repeatedly during charging at fixed intervals, therefore, the sequence returns to step S11.
If metallic foreign matter has been detected, the primary side suspends the power transmission and the secondary side does not perform battery charging (step S14).
If, for example, the user carries away the secondary side after authentication has succeeded in step S9, the secondary side fails to receive electric power and the power supply of the control unit on the secondary side is turned off. After that, even if the power supply of the secondary side is turned on again, ID number exchanging and authentication become necessary again. That is, once the power supply of the secondary side is turned off, a restart from the initial state becomes necessary. If communication between the primary side and the secondary side is discontinued or fails in step S11 or S13, therefore, the power transmission by the primary side is suspended (step S15) and the power supply of the secondary side is turned off, after which the sequence returns to step S2.
A series of processing described above is applied to charging carried out between one power transmitting device (primary side) and one power receiving device (secondary side), that is, so-called one-to-one charging.
When so-called multiple power transmission, in which one primary side charges a plurality of secondary sides, is carried out, the primary side first uses a polling command to acquire the ID number from each secondary side. The primary side then transmits authentication, control, and charging commands with an ID number specified to carry out one-to-one communication with the secondary side. Thus, the primary side can carry out authentication, control, and charging for a particular one of the plurality of secondary sides.
Japanese Unexamined Patent Application Publication No. 2011-152008, for example, describes a transfer system that performs authentication between a host (primary side) 11 and a device (secondary side) 12 before it becomes ready to transmit electric power used for data communication and device operation.
In the transfer system described in Japanese Unexamined Patent Application Publication No. 2011-152008, the host 11 intermittently transmits second electric power, which is less than first electric power transmitted when data communication is carried out, together with a response request signal. If the host 11 then receives a response from the device 12, the host 11 transmits third electric power, which is more than the second electric power and less than the first electric power, to the device 12 together with a response request signal. If a charging completion signal is included in the response from the device 12, the host 11 carries out authentication processing. If authentication succeeds, the host 11 transmits the first electric power to the device 12. If the device 12 receives the electric power from the host 11 together with a response request signal, the device 12 transmits, to the device 11, a response including a signal indicating whether charging has been completed.